The present invention relates to an oscillatory tool utilizing an oscillatory motor in a spring/mass vibration tuned system, and more specifically, to a surgical cast cutter utilizing an oscillatory motor in a spring/mass vibration tunned system.
In the past, rotating and oscillating cast cutters have been used to cut through the plaster material of casts commonly used to immobilize an extremity or a portion of the body. Rotating cast cutters and oscillating cast cutters having a large angular displacement are dangerous, not only to the patient but also to the attendant or physician removing the cast. To alleviate the dangers involved, rotating cast cutters have been abandoned and the oscillating type of cast cutter redesigned to provide an oscillation of a limited angular displacement. However, the excursion angle or displacement angle must be large enough and the frequency high enough to provide an efficient cast cutting action.
Cast cutters providing an oscillatory cutting motion are generally bulky and extremely noisy, having been developed on the principle of conversion from a rotating motor motion to oscillatory blade motion through a variety of mechanical transducing linkages and gears. The bulkiness of prior art cutters renders them unwiedly and awkward to use. In addition, and of particular importance, the noise generated in their use is extremely annoying to the operator and frightening to the patient being worked on.
U.S. Pat. No. 3,199,194 issued to Shaheen discloses a surgical cast cutter utilizing an oscillatory motor in an attempt to do away with the mechanical linkages and gearing typically required to transduce the rotating motion of a motor into the oscillatory motion of the cutting blade. In particular, Shaheen utilizes the generation of an alternating magnetic field to provide angular oscillations to the cast cutter blade. This is accomplished by directly attaching the cast cutter blade to two rotors placed on either side of the blade and further positioning two stators on opposite sides of the blade/rotor assembly. The stators are alternately energized to cause the cast cutter blade to oscillate about its axis. Despite its many supposed advantages, the cast cutter is subject to wear due to the rapid deceleration and acceleration of the stators and the armature assembly as there is no provision for absorption or diffusion of the angular energy. Additionally, the weight of the cast cutter is largely adjacent the blade resulting in poor handling characteristics. Furthermore, the torque to inertia ratio of the system is unsatisfactorily low requiring a large amount of power to overcome the inertia of the system and provide a large enough excursion angle. Additionally, electrical current must be provided to the rotating armature necessitating the use of slip ring connectors or the like which complicates the overall design and increases the overall cost of manufacturing and maintenance.
Various embodiments of oscillating systems have been utilized for such applications as optical scanners (see U.S. Pat. Nos. 3,959,673 and 4,135,119), and galvanometers (see U.S. Pat. No. 3,317,916). However, these references do not suggest or teach a tool apparatus whose desired operating frequency is approximately that of its natural frequency. The references disclose devices oscillating a very slight mass and having a relatively soft spring which only has enough stiffness to return the mass; e.g. mirror, to a center or neutral position when the devices are not energized. The torque-to-inertia ratio of these devices is relatively high such that the devices can be operated over a broad range of frequencies. The excursion angles of these devices are relatively constant and vary slightly over a broad range of operating frequencies. Furthermore, these devices, due to the slight mass, do not provide for the storage of a large amount of kinetic energy during the oscillatory motion of the system to facilitate the energy requirements of an oscillatory tool such as a cast cutter, the power requirements of the devices disclosed by the references being slight. The primary purpose of the torsion bar arrangement in these devices is to enable variable frequency of operation and accurate positioning of the tool which is being oscillated. Further, there is no attempt to absorb or diffuse angular energy to reduce wear. For this and other reasons the references do not teach the use of a torsion bar arrangement for an oscillatory tool utilizing an oscillatory motor in a spring/mass vibration tuned system and in particular a cast cutter which utilizes such a system.
The present invention solves the above indentified problems and others associated with the prior art.